I. Field of Invention
The invention generally relates to wireless communication systems, and more particularly to satellite and aerial communication system that utilize orthogonal multiple access techniques.
II. Description of the Related Art
Various communication systems, including satellite and aerial, have been developed over the years. Orthogonal Code Division Multiple Access (OCDMA) is one technique that can be used in satellite communication systems. Code Division Multiple Access (CDMA) is characterized by the use of spread spectrum modulation technique that provides separate user or user traffic signal channels. In typical CDMA based satellite systems, signals from different terminals are not synchronized. As a result, the signals can arrive out of code phase and cause interference with each other.
In OCDMA based satellite systems, the signals transmitted from different terminals are timed such that they arrive synchronously, and are in code phase with each other or have code synchronization, at the receiver. Particularly, each user terminal is assigned orthogonal Walsh codes that are used to distinguish different terminal transmissions. With synchronization in frequency and in time, there is generally no or very little cross-interference among the different signals received from terminals since the cross-correlation approaches zero. This achieves higher bandwidth efficiency due to the significantly reduced interference.
Orthogonal multiple access may also be achieved using other modulation schemes such as Orthogonal Frequency Division Multiple Access (OFDMA). In OFDMA the available bandwidth is divided into a number of frequency bands referred to as tones. These tones are orthogonal to each other in the sense that after matched filtering at the receiver, no interference is observed from other tones. Different users are assigned a subset of the available tones. A guard time, where the cycle prefix of OFDMA symbol is inserted, helps maintain orthogonality among different tones as long as it is larger than the largest differential delay among users. In systems with large differential delays among terminals such as Geostationary satellites or communication from airplanes to a ground station the guard time needs to be excessively large resulting in efficient use of bandwidth. Therefore, even in OFDMA one would require a certain degree of timing synchronization among different terminals in order to reduce the length of the cyclic prefix. Frequency synchronization among different users received signal is needed in OFDMA as well as OCDMA.
On the forward link of a satellite communication system, orthogonality among different codes is effectively maintained because the signals originate at the same location, namely at the hub or gateway that provides communication service to one or more remote terminals. Where several gateways are used throughout a communication system, they are generally configured to use a common timing source, such as the phase of signals detected from Global Positioning System (GPS) satellites, which employ a form of what is referred to as Universal Time. Alternatively, gateways can be in communication with each other and/or a timing signal reference may be used to provide a synchronization mechanism.
OCDMA/OFDMA techniques may also be used in the return or reverse link paths of a satellite and aerial communication systems, that is, for signals transferred from terminals to a hub or gateway or from airplanes to a ground station. The use of OCDMA in the return signal is disclosed in co-pending U.S. patent application Ser. No. 10/603,421, filed on Jun. 23, 2003 under the title “Orthogonal Code Division Multiple Access on Return Link.” Generally, for signals being transferred by terminals on the return link, there is no common synchronization mechanism. Accordingly, signals transmitted from different terminals may arrive asynchronously at the gateway(s) due to their different propagation delays. Therefore, while satellite communication systems can easily incorporate OCDMA or and OFDMA for use on forward links, it is more difficult to use this technique on the reverse links. Moreover, mobility creates additional problems in synchronizing the time bases and frequencies of the transmit signals in the reverse link paths.
Therefore, there is need for a more efficient and/or effective orthogonal multiple access, such as for example OCDMA or OFDMA, in the return signal paths of a satellite communication systems.